The invention relates to a high efficiency drive circuit for driving an FET having an insulated gate (hereinafter referred to as insulated gate type FET) such as a Metal Insulator Semiconductor Field Effect Transistor (MISFET) and a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) by impressing a boosted voltage on the gate of the FET.
Insulated gate type MISFETs and MOSFETs are widely used as switching elements in controlling disk devices such as CDs, MDs and DVD-ROMs. The resistance of a MOSFET, for example, when it is turned on (hereinafter referred to as ON resistance) greatly affects the power conversion efficiency of an appliance which uses the FET. Therefore, in order to impress a higher voltage on the gate of a MOSFET than that of a power supply so as to lower the ON-resistance of the MOSFET and maintain a high power conversion efficiency thereof, a charge pump circuit is used as a power supply to drive the gate.
FIG. 1 illustrates a conventional MOSFET drive circuit, which boosts up the source voltage Vcc of a power supply 10 by means of a double charge pump circuit 20, and supplies the boosted voltage to an n-type load MOSFET Qm via a gate control circuit 30. The MOSFET drive circuit is incorporated in a semiconductor integrated circuit (IC).
The double charge pump circuit 20 includes a p-type MOSFET Q1, a p-type MOSFET Q2, inversion buffer circuits B1-B3, condensers C1 and C2, and an oscillator circuit OSC.
The double charge pump circuit 20 boosts up the source voltage Vcc by first turning on the MOSFET Q1 and turning off the MOSFET Q2 when the output level of the oscillator circuit OSC is high and the output level of an inversion buffer circuit B1 is low and the output level of an inversion buffer circuit B2 is high. The output of the inversion buffer circuit B3 is then at L level. Hence, the condenser C1 is charged towards Vcc.
When the output of the oscillator circuit OSC is low, the output of the inversion buffer circuit B1 is high, so that the MOSFET Q1 is turned off, and the output of the inversion buffer circuit B2 is low, so that the MOSFET Q2 is turned on. The output level of the inversion buffer circuit B3 is then high. As a result, the condenser C2 is impressed with the voltage of the condenser C1 plus the high output voltage of the inversion buffer circuit B3 (which equals the Vcc), so that the condenser C2 is charged towards 2Vcc.
Such charging process is repeated at every inversion of the output of the oscillator circuit OSC from H to L level and from L to H level, bringing the charged voltage of the condenser C2 to 2Vcc. This voltage is supplied to the gate of the load MOSFET Qm via the gate control circuit 30. The voltage of 2Vcc is presumably large enough to drive the load MOSFET Qm, so that the load MOSFET Qm has a very small resistance as it is fully turned on by the voltage.
The prior art MOSFET drive circuit as shown in FIG. 1 has a comparatively simple structure to provide a necessary boosted voltage. The external condensers C1 and C2 for the IC are not costly.
The gate circuit of the MOSFET is electrically insulated from the drain and the source of the MOSFET by an insulating oxide film, so that in principle the gate does not require any electric power, so long as it is provided with a predetermined electric potential, and hence it has been believed that it requires only a simple drive circuit as shown in FIG. 1.
In actuality, however, a current flows through the gate charging the floating capacity thereof every time the load MOSFET Qm is switched. Consequently, when the area of the gate is increased to lower the ON-resistance, or when the switching frequency of the load MOSFET Qm such as a PWM driver for example, energy consumption due to the switching becomes large and lowers the output voltage of the MOSFET drive circuit. That is, the circuit becomes unable to provide a required gate voltage.
Under the lowered output voltage, the load MOSFET Qm cannot be fully turned on and exhibits a non-negligible resistance, if the load MOSFET Qm maintains its switching operation. Consequently, the power conversion efficiency of the appliance having the load MOSFET Qm is degraded.
A chopper type power supply is also known to serve as an alternative voltage booster. A chopper type power supply can maintain its output voltage at a constant level, and has a superb constant output-load characteristic. It normally requires a costly coil and a Shottkey diode as external control elements therefor.
In short, it has been difficult to provide a cost effective high-efficient drive circuit due to the fact that conventional MOSFET drive circuits utilizing a charge pump circuit are cost-effective but have a poor output-load characteristic for a MOSFET Qm and that a MOSFET drive circuit utilizing a chopper booster circuit has a good output-load characteristic but requires expensive external elements.
In accordance with one aspect of the present invention, a drive circuit for driving an insulated gate type FET (hereinafter referred to as insulated get FET drive circuit) comprises: an oscillator circuit providing an oscillatory output; a charge pump circuit for boosting up a voltage input thereto in response to the oscillatory output of the oscillator circuit; and comparison means for comparing the output voltage of the charge pump circuit with a preset reference voltage and for outputting the result of the comparison; wherein the charge pump circuit includes: at least one charge pump unit, concatenated if more than one charge pump unit exist, each charge pump unit having a switch turned on/off by the oscillatory output of the oscillator circuit and a condenser connected between the switch and the oscillator circuit; and a charge pump unit in the output stage of the drive circuit having a switch turned on/off by the oscillatory output of the oscillator circuit and a condenser connected between the switch and a source of predetermined electrical potential; and wherein the charge pump unit in the output stage of the drive circuit is further concatenated with the at least one charge pump unit; wherein the charge pump circuit supplies the output voltage thereof to the gate of the FET; and wherein the comparison means controls stopping/starting of the oscillation of the oscillator circuit in accordance with the comparison.
Thus, the charge pump circuit of the invention can provide a sufficiently large voltage as required to the gate of the insulated gate FET in a stable manner by operating the oscillator circuit such that the charge pump circuit is intermittently operated, i.e., the charge pump circuit is stopped and started intermittently. Accordingly, the drive circuit can maintain the ON-resistance of the insulated gate FET at L level, thereby maintaining a high power conversion efficiency of the FET. In this arrangement of the charge pump circuit, integration of the elements of the circuit on an integrated circuit board is easy. Condensers may be fabricated at low cost if they are provided as external elements.
It should be noted that the comparison means may have a voltage divider for generating a divided voltage from the output voltage and a comparator for comparing the divided voltage with the preset reference voltage. Thus, the output voltage may be set to a desired level by changing the division ratio of the voltage divider.
In addition, the phase of the output voltage of the oscillator may be chosen such that the switch of the charge pump unit in the output stage is turned off when the oscillator circuit is stopped, so that the condenser of the charge pump unit in the output stage is quickly charged when the oscillation is resumed subsequently.
Also, the switch of the drive circuit may be a P-channel MOSFET and the gate of the switch is driven by the output of an inversion buffer circuit. Furthermore, the charge pump circuit along with the charge pump unit in the output stage may include three charge pump units.
In accordance with another aspect of the present invention, the comparison means may be a comparator coupled to the oscillator circuit.
Also, in accordance with a further aspect of the present invention, a method for providing power to an insulated gate FET comprises the steps of: providing an oscillatory output; generating an output voltage by boosting up an input voltage in response to the oscillatory output; supplying the output voltage to the gate of the FET; comparing the output voltage with a preset reference voltage; and controlling the stopping/starting of the oscillatory output in accordance with the comparison of the output voltage with the preset reference voltage. The method also may include the following steps of generating a divided voltage from the output voltage, comparing the divided voltage to the preset reference voltage, and controlling the stopping/starting of the oscillatory output in accordance with the comparison of the divided voltage with the preset reference voltage.